This is a proposal to analyze electrophysiologic recordings taken directly from the cerebral cortex of epilepsy patients to test our hypothesis that seizures arise from a pathological epileptic system. We will develop and apply coherence analysis to EEG recordings that are obtained from arrays of electrodes that are implanted over the cortex of epilepsy patients as part of their work-ups for determining the source of their intractable epilepsy. Current techniques employed for the evaluation of seizure foci are subjective, imprecise, and ineffective a large portion of the time, leaving many of these patients postoperatively unable to work or otherwise carry on a normal life style. We plan to take recordings from 75 epilepsy patients and 25 control patients over a four-year period with the goal of determining the optimal analysis strategy to identify areas of cortex that have become part of the epileptic system, and differentiate them from normal and eloquent cortex. This will involve comparing standard clinical evaluation techniques to our new measures of lateral coherence, which should increase our understanding of the spread of epileptic seizures and the underlying neuropathology associated with seizure activity. At the time of surgery the coherence patterns will be interactively displayed directly on the cortical surface in real-time by means of intra-operative digitized video images. This technique should also make it possible to reduce the surgical morbidity associated with epilepsy surgery and increase its efficacy, thereby allowing a greater proportion of intractable epilepsy patients to realize self-sufficient and productive lives.